The need to retain and retrieve large volumes of digital data has resulted in the need for data storage devices having ever increasing amounts of storage capacity. Data can be stored, of course, on cassettes, floppy disks, diskettes, hard disks, optical disks, capacitive disks and the like. However, the greater the amount of memory available, the more difficult it becomes to accurately access the information from multiple input computers with rapid access times and with maximum system fault tolerance.
Other prior art systems utilize a plurality of data storage mediums having a robotic arm control which retrieves stored disks or cassettes from a library and places them in the recorder. For example, data storage media are transported from a storage area to a transducer mechanism such as is utilized for magnetic tape, floppy disk, hard disk and other mass storage devices. After the transducer has completed its use of a particular storage medium, the medium is returned to the storage area to make room for another unit which is retrieved, transported to and placed in the transducer.
Some prior art data storage systems include optical storage wherein a plurality of cartridge storage slots are positioned in diverse locations and orientations and use both vertical and horizontal retrieval systems. Still other systems utilize disk storage and have a disk transport assembly which moves in parallel with the array of the disks independently of the disk retainer. The transport assembly is moveable between a first position operative to have a disk transferred between the array of disks and a disk carrier and a second position operative to have a disk transferred between the disk carrier and the disk retainer.
In order to improve fault tolerance, the prior art has provided disk files comprising a number of stacks having alternate data paths provided using crossbars. This provides some fault tolerance since if one data path fails another can be used. Some prior art systems disclose fault tolerant computer systems including a robot maintenance means which is used to replace faulty modules and still others disclose an optical storage/retrieval including a random access arm supplying several users.
Still, these systems are inadequate. No matter what drive and storage medium is used in any given installation, use of the system invariably places demands on mass storage that soon exceed the available capacity of the system. Often these demands are not foreseen when the system in initially planned and the need then arises to expand the system. However, it is difficult to expand such a system in an economical manner. Further, it is difficult in such systems to have parallel operations whereby several computers may be simultaneously accessing the mass storage library.